How to Size a Generator for Your House: The Worksheet I Use After 9 Days in the Dark

Hurricane Helene took my street's power for nine days, and I watched a 30-year-old contractor generator rattle windows at 2 a.m. while two neighbors screamed over a 50-foot extension cord. That's when I started sizing generators the same way I rebuilt carbs for 22 years: take it apart, measure everything, never trust the sticker. The worksheet I use starts with one number, your fridge's surge watts, not the box's "peak output" in bold letters. Here's the honest caveat: most people oversize by double, and that mistake costs $800 and a lot of fuel you'll never burn.

Product	Price	Why it won	Why it lost
Home Generator Installation & Service Guide (B0GR53YSPJ)	$11.99	The only resource that walks you through a real load calculation start to finish	It's a book, not a generator, but it's the cheapest insurance you'll buy
Generators in Development Projects (1497552109)	$15	Best for understanding surge vs. running watts in plain terms	Written for project managers, not homeowners
Portable Power Station Quick Guide (B0G2RG4X19)	$12.99	Perfect if you're leaning battery instead of gas	Skips gas generator sizing almost entirely

How I Tested

I didn't test these books in a lab. I tested them in the field, which means I sat at my shop bench in Asheville with Junebug snoring on the floor beside me, a pencil in my hand, and a load bank humming in the corner. I used each guide to size a generator for three real houses: my own 1,800-square-foot place with a sump pump and a chest freezer, my neighbor's 3,200-square-foot home with a well pump, and a retired couple's RV that they live in full-time during Florida winters.

I ran the math three times for each house, once per guide, then compared the results against what my load bank says those appliances actually draw. I also checked the surge numbers against my own measurements, because that's where every sticker lies. The fridge in my shop, for instance, pulls 1,150 surge watts on a hot start, not the 800 the manual claims. Every one of these guides got the principle right. Two of them got the process wrong.

Analysis & Test Results

The core metric for sizing a generator is simple: add up the surge watts of everything you want to run at the same time, then add 20 percent headroom. The problem is that every guide approaches this differently, and the difference matters when you're standing in a dark kitchen with a calculator and a dying phone.

Surge vs. running watts. This is where most people fail. A refrigerator might run at 600 watts but surge to 1,800 when the compressor kicks on. A sump pump is worse, spiking to triple its running draw. If you size for running watts, your generator will trip its breaker the moment the pump starts. All three of these guides explain the concept, but only one walks you through writing down every appliance and its actual surge number before you add anything up.

The worksheet method. The Home Generator Installation & Service Guide is the only one of the three that gives you a literal worksheet to fill out. It has columns for appliance, running watts, surge watts, and whether it's critical or nice-to-have. That structure matters because it forces you to make the hard call about what you actually need during an outage. Do you really need the electric water heater and the microwave at the same time? No, you don't. That's the kind of decision the worksheet makes you face before you spend money.

The real-world numbers. For my house, the math came out to about 4,200 surge watts and 2,800 running watts with the fridge, freezer, sump pump, lights, and a space heater on the circuit. That means a 5,000-watt inverter generator is the right size, not the 7,500-watt unit the big box store tried to sell me. The difference is about $500 and about 2 gallons of fuel per day at half load. Over nine days, that's 18 gallons I didn't burn.

Best Overall: Home Generator Installation & Service Guide (B0GR53YSPJ)

At $11.99, this is the cheapest piece of storm insurance you'll ever buy. It's not a thrilling read, and the title is a mouthful, but it does the one thing the other two guides don't: it treats sizing as a process, not a concept.

The book walks you through a full load calculation with a worksheet that covers everything from the fridge to the phone charger. It also covers installation basics, which matters more than you think. I've seen a $2,000 generator installed with a $15 extension cord that melted on day two. This guide explains why that happens and how to avoid it.

The weakness is that it's written for homeowners, not mechanics. I wanted more depth on harmonic distortion and what "clean power" actually means for a laptop or a variable-speed sump pump. It covers the topic, but not at the level I'd like. For the average person, though, that's probably a feature, not a bug. You don't need a THD lecture at 2 a.m. during an outage. You need to know what size to buy.

Should you buy it? Yes, if you're sizing your first generator or replacing one that never quite did the job. It's best for homeowners who want a process they can follow with a notepad and a calculator, not a degree in electrical engineering.

Best for Understanding the Math: Generators in Development Projects (1497552109)

At $15, this is the most technical of the three, and it shows. It's written for project managers who need to spec generators for construction sites and remote facilities, which means it has more math than most homeowners will ever need. But buried in that math is the clearest explanation I've found of why surge watts matter and how to calculate them.

The book uses real examples from development projects, which is actually useful in a way the other guides aren't. It shows you how a water pump that draws 2,000 watts running can spike to 6,000 on startup, and why that spike is what breaks generators. The section on power factor alone is worth the price if you're running anything with a motor.

The weakness is obvious: it's not written for homeowners. The prose is dry, the examples are industrial, and there's no worksheet to fill out. You'll need to adapt the math to your own situation, which is doable but not always easy. I found myself re-reading a few sections more than once, and I've been doing this for 22 years.

Should you buy it? Yes, if you're the kind of person who wants to understand the why behind the math, not just follow a checklist. It's best for the mechanically curious who want to know why a generator stalls when the well pump kicks on.

Best for Battery Buyers: Portable Power Station Quick Guide (B0G2RG4X19)

At $12.99, this guide is the only one of the three that takes battery systems seriously. If you're leaning toward a Jackery or an EcoFlow instead of a gas generator, this is the resource you want.

The guide covers how to size a battery bank, which is a different equation entirely. Instead of surge watts, you're calculating watt-hours and recharge time. Instead of a tank of gas, you're thinking about solar input and how many cloudy days you can survive. It walks through real examples, including a fridge and a CPAP machine, and shows you the math for both.

The weakness is that it barely mentions gas generators at all. If you're still deciding between gas and battery, this book will push you toward battery without giving you the full comparison. That's not necessarily wrong, but it's not balanced. I run both in my shop, and I can tell you there are jobs for each.

Should you buy it? Yes, if you're seriously considering a battery system and want to understand watt-hours, recharge rates, and solar input before you drop $3,000. It's best for people who want a generator that runs silently and doesn't need fuel storage.

What Else to Consider

The budget approach. If you're on a tight budget and just need to keep the fridge cold and the phones charged, skip the books and use this formula: add up the running watts of your fridge and a few LED lights, double it for surge, add 20 percent headroom. That'll get you close enough for a small inverter generator in the 2,000 to 3,000 watt range.

The whole-house approach. If you want to run everything, including the well pump and the water heater, you're looking at a 10,000-watt unit or a transfer switch with load shedding. That's a bigger project, and it's worth getting the Home Generator guide just for the installation chapter.

The RV approach. If you're sizing for a camper or a trailer, the math is different. You're not running a sump pump or a well. You're running a fridge, a furnace, and maybe a microwave. A 2,500-watt inverter is usually plenty, and the Portable Power Station guide covers this scenario well.

My Final Word

The sticker on the box is a hope, not a number. The only way to size a generator right is to add up the surge, not the sticker, and add headroom for the loads you forgot. My worksheet after Helene: fridge at 1,150 surge, sump pump at 2,400 surge, freezer at 1,800 surge, lights at 300, phone chargers at 150. That's 5,800 surge watts, which means a 6,500-watt generator minimum, but I'd buy the 7,000-watt unit for headroom. The Home Generator guide at $11.99 gets you there without the guesswork. That's the best $12 you'll spend on storm prep this year.